Seth Rogen's new comedy Neighbors casts him in what seems like a new light. His character Mac is married with a kid, works a 9-to-5 job and lives in the suburbs. Mac ought to be happy, but he can read the writing on the wall: He's no longer cool, and he's turning into just another boring old square. Only amplifying that feeling is the arrival of a party-hearty fraternity next door, led by the buff, smug Teddy played to pretty-boy perfection by Zac Efron. Through Teddy's eyes, Mac sees himself for the schlubby, domesticated loser he's become.

Neighbors is hilarious in lots of ways, but one of its central jokes is that Rogen, best known for portraying immature stoner screw-ups in Pineapple Express and Knocked Up, is now getting to the age where he plays actual men, not boys. Rogen, who turned 32 last month, has always tried to have it both ways. Yes, he remains famous for his love of weed and his massively dorky laugh. (Both habits are on display in Neighbors.) But as his films have long demonstrated, he's fought to transcend the loser/stoner persona, coming across as an intriguing blend of sympathetic romantic figure, heartfelt softie and reasonably responsible adult (who also happens to be a loser/stoner). His career appears to be an ongoing experiment: Can a slacker grow up without becoming lame?

Though Rogen had a very small part in Anchorman, he got his first taste of movie stardom with 2005's The 40-Year-Old Virgin. (The film was written and directed by Judd Apatow, who executive produced the cult TV show Freaks and Geeks where Rogen got his start.) In The 40-Year-Old Virgin, Rogen played one of Steve Carell's coworkers, an overgrown, going-nowhere man-child. Rogen was funny in the role but also a little too perfect: With his hefty build, 'fro-tastic hair and goofy demeanor, he seemed destined to play wise-ass sidekicks for life.

Instead of becoming the Rob Schneider for Team Apatow, however, Rogen recast himself as a leading man, albeit one barely removed from his Virgin bozo. Reuniting with Apatow for Knocked Up, Rogen portrayed Ben Stone, a loser trying to put together a porn site, who impregnates a driven, beautiful TV reporter (Katherine Heigl) after a forgettable, drunken one-night stand.

The film's comedic conceit was that this slovenly layabout couldn't possibly land a woman like Heigl. But Apatow understood that a wellspring of sincerity and warmth bubbled underneath Rogen's dipshit exterior, and so Knocked Up became the story of how a bozo finally develops into a sensitive adult. Ben remained a hopeless slob, but deep down he was a good guy, which made us root for him. Utilizing his patented nonchalant acting style—Rogen often looks like he dragged himself out of bed right before the cameras started rolling—the burgeoning star demonstrated a rare skill—playing somebody who was decent but lazy, the poster child for good intentions but squandered opportunity.

Knocked Up was one of 2007's biggest hits, and pretty soon Rogen was everywhere: cameo in Step Brothers, voice actor in kids' movies like Horton Hears a Who! (Among his other talents, he's uniquely suited to animation: He comes across as a big kid himself, and his voice is full of boundless, uncynical enthusiasm.) At the same time, Rogen also wrote with his producing partner Evan Goldberg, developing the screenplays for Superbad,Pineapple Express and The Green Hornet. Still in his mid-20s and taking advantage of his connection to Apatow, one of comedy's rising auteurs, Rogen was a bona fide Hollywood player—an assertion that the actor-writer would probably mock. "I'm not really a goal-oriented guy," Rogen told The A.V. Club with a laugh in 2008. His greatest aspiration, he said, was "to not make any giant-piece-of-shit movies."

Rogen talks a good game, but he's actually far more focused and ambitious than his stoner-bro persona suggests. Discussing the action-comedy Pineapple Express with The A.V. Club, he bristled at the idea that the film was a "stoner comedy." He said, "When you say 'a stoner movie,' I picture something that only people who smoke weed like, like a Cheech and Chong movie, or something in that vein." Just about every interview with Rogen mentions his love of pot—and, of course, his movies feature him smoking weed—but he's worked hard not to be thought of as just some blunt-loving idiot. "Clearly [Goldberg and I] found some success," Rogen said in the same interview, sounding somewhat defensive. "We can't be totally brain-dead yet."

To the contrary, his stoner vibe has become a deft Trojan horse. His onscreen roles often capitalize on the same lowered expectations his modest, silly offscreen demeanor elicits. His movie characters tend to follow the Knocked Up model. Whether it's Zack in Zack and Miri Make a Porno or Britt Reid in The Green Hornet, Rogen has continually played the lovable slob. But unlike previous lovable slobs such as John Belushi or early Bill Murray, he doesn't possess a lot of anger or edge—he's too laidback to rail against the system. For all of their profanity and bad behavior, Rogen's characters are mostly just carefree guys, proudly emulating the toking, just-hanging-out hero the Dude in one of Rogen's favorite films The Big Lebowski. Maybe this explains why 2009's dark comedy Observe and Report failed to connect with audiences: Rogen's creepy, unhinged mall cop was an unwelcome departure for his fans.

As Rogen's stature has grown, he's gotten more serious—although, thankfully, without becoming one of those insufferable stars straining for credibility. (This separates him from his good buddy James Franco.) He produced the cancer comedy-drama 50/50, in which he played the stereotypical sidekick/buddy to Joseph Gordon-Levitt. When Rogen appeared in Apatow's bid for a more mature James L. Brooks-style of character comedy, Funny People, he ably negotiated that film's sometimes-awkward mixture of pathos and laughs. In the underrated, bittersweet romantic drama Take This Waltz, Rogen gave his most affecting performance starring opposite Michelle Williams, a young married woman who's starting to have doubts about how happy she is with her complacent, infantile husband (Rogen). And even in This Is the End—an R-rated comedy that Rogen also co-wrote and co-directed—he manages to balance jokes about gigantic devil penises with more heartfelt commentary about the awkwardness and poignancy that occur when childhood friends drift apart. (And it's worth pointing out that This Is the End's best running joke is how useless Hollywood actors would be during the Apocalypse, Rogen essentially diminishing his own celebrity and self-importance.) The guy's not trying to do Beckett, but likewise his creative choices aren't brainless: There's a sincerity—a warm, emotional purity—he brings to his roles that feels effortless.

That probably was never more evident than earlier this year when he appeared on Capitol Hill to promote increased funding to combat Alzheimer's, a disease his mother-in-law has been afflicted with for nine years. At the outset of his testimony, he made a predictably self-deprecating marijuana joke before telling his personal story about how Alzheimer's had stripped away everything that had made this woman so special. Nervous, wielding that dorky laugh as a defense mechanism, Rogen once again looked like a big, goofy kid in that room of politicians and suits. He may not have been fully confident, but genuineness has always been his ally, and as with his films, his willingness to embrace earnest sentiment in the moment was disarming.

Now with Neighbors, his career comes full circle. As Rogen himself has acknowledged, his new film could be a sequel to his star-making turn in Knocked Up. "It didn't escape us as we were making it," he told the Associated Press. "If you caught up with [Ben Stone] the next year, there's a chance this is kind of a version of what his life would be like." Just as Ben stumbled into adulthood with the unexpected arrival of his first child, Neighbors' Mac thinks he's ready for grownup life with the family and office job, only to discover that he can't quite let go of the wild-and-crazy lifestyle he once enjoyed.

Rogen's characters often grapple with letting go of childish things, but in Neighbors, which he produced, but didn't write, he twists that knife further by playing the character who's the over-the-hill sucker. For years, he's been the young dude with the father figure or older mentor—in Neighbors, he's the dad. The movie's doubly funny because Efron represents all the beautiful, hunky pinups—the traditionally handsome leading men—that have been the antithesis of Rogen's flabby sloth. But in a sign of Rogen's eternal, laidback niceness, Neighbors doesn't go out of its way to mock the kid. In the end, the two guys even become friends. That's always been Rogen's appeal: Deep down, we all end up just really liking the guy. Even his laugh.

Tim Grierson is Playboy for iPhone's critic-at-large. He is the vice president of the Los Angeles Film Critics Association and the author of "FilmCraft: Screenwriting." Follow him on Twitter @timgrierson